1. Field of the Invention
The present invention relates to interferometers, and more particularly, the use of reference signal coherence and time division multiple access to perform interferometry by encoding reference and signal data into data streams and decoding measurement parameters such as S21 parameters from the received data stream. In addition to measuring S21 parameters, the present invention extracts measurement trigger timing and additional metadata from the data stream.
2. Description of Related Art
Near-field measurement of an antenna phase front requires a spatially distributed set of complex S21 RF transmission measurements. The phase front measurement is made using a spatially scanning microwave interferometer. The interferometer has two paths, a signal path that includes an antenna under test and a test probe, and a separate reference path. The complex ratio of the signal path signal to the reference path signal gives the desired S21 transmission ratio.
These near-field measurement systems become expensive as measurement rates increase. Additionally, all characteristics of the transmitted wave are reduced to a single measurement of the S21 transmission ratio at each spatial location, and potentially useful metadata is lost.
FIG. 1 depicts the architecture of a near-field measurement system typical of the prior art. A Radio Frequency (“RF”) source 102 is split 104 into a signal path 106 and a reference path 108. One input of the receiver 110 is connected to the reference path. The signal path has a transmit 112 and receive 114 element, and the receive element connects to a second input of the receiver 110. Signal 109 shows the time domain waveforms of an example RF source signal that has passed through the reference path and signal 107 shows the time domain waveform of the same RF source signal after passing through the signal path. As shown, the signal path waveform 107 has an amplitude and phase shift relative to the reference path signal. Within the receiver the signals from the signal path and reference path are converted into complex form by a digital quadrature down converter or by a Hilbert transform and are input into a complex divider 116. When an external trigger 118 is input into the receiver, the result of the complex division is present as an output of the receiver 120. S21 is a complex number representing the convolution of the two antenna characteristics. This number is used to characterize the antenna under test, which can be the transmit element 112 or the receive element 114. The information is stored in a data array position corresponding to the relative spatial location of the receive element 114 at the time of the trigger. By physically moving the transmit and receive elements relative to one another and obtaining an S21 measurement at each location triggered by the trigger 118, an amplitude and phase front for different spatial locations relative to the transmit element can be mapped.